Adapters to A-Mount

[bitsnpieces]

I discovered adapters when looking into the NEX series and how with an adapter, you can get vintage lenses on them, or like A to E mounts, etc.

So it's got me thinking, are there any adapters from other branded cameras, to Sony? I currently have an 18-250mm, but I want to get one which has a fixed aperture of at least f2.8 - I guess 3.5 or so isn't going to kill me, but f2.8's nice right?

Anyways, what's the cheapest available, and is there an adapter for it? *fingers crossed*